1. Field of the Invention
This invention relates to screw rotor construction for use in oil flooded screw compressors to be applied to oil cooled equipments such as screw compressors, vacuum pumps, inflaters or the like.
2. Prior Art
In determining the tooth profiles of screw rotors of an oil flooded screw compressor, namely, the tooth profiles of flank portions on the trailing sides of rotors in the direction of rotation indicated by arrows in FIG. 5, it has been the general practice to determine firstly a suitable function F(x) which expresses the tooth profile of the tip end on the trailing side of a male rotor 11. Once the function F(x) is determined, there is automatically obtained a function f(x) which expresses the tooth profile on the trailing side of a female rotor 12 as generated by the tip end of the male rotor 11. Nextly, a suitable function G(x) which expresses the tooth profile of the female rotor 12 from the surface of the function f(x) to its addendum circle 13 is determined. This function G(x) automatically determines the function g(x) of the surface on the trailing side of the male rotor 11, which is generated by the female rotor profile of the function G(x).
The surface of the function G(x) has its center of curvature located either on the trailing side (Japanese Patent publications 60-41238 and 56-17559) or on the pitch circle on the trailing side (Japanese Laid-Open Patent Application 60-153486).
The tooth profiles on the trailing sides of screw rotors have great influences on the performance quality of a screw compressor or the like, and are desired to be small in seal line length and blow hole area.
However, as shown in FIGS. 5 through 9, the seal line length and blow hole area are contradictory to each other in nature, namely, they are in the relationship that one is increased by reduction of the other.
The broken lines in FIG. 5 indicate the loci of a seal point between the rotor tooth surfaces of the functions F(x) and f(x) and a seal point between the rotor tooth surfaces of the functions G(x) and g(x), which are draw in when the female and male rotors 12 and 11 are rotated in the directions indicated by the arrows in the figure. Namely, as the rotors are rotated, the seal point between the rotor tooth surfaces of the functions F(x) and f(x) is shifted from point B to point A, while the seal point between the rotor tooth surfaces of the functions G(x) and g(x) is shifted from point B to point C. Point C, which is located on addendum circle 13 of the female rotor, is the last point of contact between the rotor tooth surfaces G(x) and g(x). Indicated at 15 and 14 are pitch circles of the female and male rotors 12 and 11, respectively.
Shown in FIG. 6 is the relationship of the rotational angle .phi. of the rotors from a reference point, namely, from point A where the seal point between the rotor tooth surfaces of the functions F(x) and f(x) is located on a line which connects the centers of the two rotors, or of the distance of the seal point from the reference point in the axial direction of the rotors, with the distance of the locus of displacement of the seal point from the reference point as measured on the plane xy, that is, on the plane of the drawing. Points a and c in FIG. 6 correspond to the points of large letters A and C in FIG. 5, and point b corresponds to the point of the larger letter B. The length of the seal line is equivalent to the length of the curves ab and cb in FIG. 6, and determined mostly by the location of the point B. The point B in FIG. 5 can be shifted to position B' by reducing the distance between point E (the point at which f(x) meets G(x) and the center of the female rotor O.sub.F. Since B' is closer to the point A than Point B, the point b in FIG. 6 is shifted to point b' as indicated by one-dot chain line to form a shortened seal line. In FIG. 6, the reference characters .phi..sub.B and .phi..sub.B' indicate the rotational angles of the rotors at points B and B', respectively, at which the female and male rotors 12 and 11 first engage with each other.
As seen in FIG. 7, the respective notations have the following definitions.
l.sub.MF : Minimum distance between the female and male rotors; PA0 l.sub.MP : Minimum distance between the male rotor and cusp P (the crested portion of the motor chamber wall); and PA0 l.sub.FP ; Minimum distance between the female rotor and cusp P.
Shown in FIG. 8 are variation curves of the above-mentioned distances l.sub.MF, l.sub.MP and l.sub.FP plotted against the rotor rotation angle .phi. or the distance in the axial direction of the rotors on the horizontal axis and the distance on the vertical axis. Plotted in FIG. 9 are smaller portions of the distances of l.sub.MP and l.sub.MF +l.sub.FP, in which the horizontal and vertical axes have the same meaning as in FIG. 8 and, for the convenience of illustration, the distance l.sub.FP is plotted on the lower side on the basis of the distance curve for l.sub.MP. In this instance, the blow hole has an area which is circumvented by points s, t, u and v in FIG. 9. The angle .phi..sub.o is the angle where the tip end of the male rotor reaches the cusp P, and takes a certain fixed value. In the same manner as explained in connection with FIG. 6, when the position of the point B in FIG. 5 is shifted to B', the angle .phi..sub.B changes to .phi..sub.B' as a result shifting the distances l.sub.MP and l.sub.FP as indicated by the one-dot chain line and the points s, t, u to s', t' and u', respectively.
Accordingly, in case of the conventional tooth profile where the center of curvature of the surface of the function G(x) is located on the trailing side, the shift to .phi..sub.B' of the rotor rotation angle .phi..sub.B, at which the female and male rotors disengage from each other, is reflected by a shortened seal line but at the same time by an increased blow hole area as a result of substantially parallel shifts of the curves st and tu of the distances l.sub.MF and l.sub.FP.
On the other hand, as shown in FIG. 10, in case of an oil flooded type compressor or the like, the tooth grooves 17 and 18 of the female and male rotors 12 and 11 are sealed off from each other by oil 16 which intervenes the two rotors. The sealing effect of this oil portion 16 is enhanced as the oil pressure is generated by wedge effect which is produced according to the relative speed of the female and male rotors 12 and 11. Namely, when the relative speed of the female and male rotors 12 and 11 is zero, the oil pressure at the oil portion 16 becomes smaller, lowering the sealing effect of the oil portion 16.
Therefore, there is a problem that, when the center of curvature of the tooth surface of the function G(x) is located on the pitch circle of the female rotor, the relative speed of the male and female rotors at the seal point becomes zero, impairing the afore-mentioned sealing effect.